Radar systems are used to determine the distance to moving or stationary objects (target objects) and/or to determine the (relative) velocity of moving or stationary objects (target objects) in different observation ranges (distance ranges). The main areas of application for radar systems are as a rule observation ranges with long distances between the radar and the target object(s) ("long-distance range", e.g., a distance of up to 150 km or 300 km depending on the application), for instance in aviation for flight safety or for navigation purposes. More recently, there are also applications for radar systems in observation ranges with a very short distance between radar and target object(s) ("close range", e.g. up to 20 m or 250 m depending on the application), for instance in motor vehicle applications for determining the distance to motor vehicles travelling in front or behind, or approaching from another direction, or to other reflection objects, and/or the relative velocity of motor vehicles travelling in front or behind, or approaching from another direction, or to other reflection objects. The analog, high-frequency signal (transmission frequency in the GHz range, typically between 18 GHz and 94 GHz) generated by the transmitter unit of the radar system by means of an oscillator and emitted from a (transmission) antenna is detected by the (receiving) antenna of the receiver unit of the radar system after having traversed a transmission path and after having been reflected at the target objects (reflection objects) situated in the observation range and this received signal (reflection signal) is evaluated after signal processing (further processing) with respect to propagation time and/or frequency shift or phase shift. The required distance and/or velocity information can be obtained from this.
The two commonly used radar systems, the pulse radar system and the FMCW radar system, differ with respect to the measuring principle, in particular in the generation of the transmission signal and in the variation of the transmission signal (over a period of time):
In the pulse radar system, the transmission signal is cyclically interrupted, i.e., transmission pulses are emitted with a specific pulse-on time from the transmitter unit. In the pulse-off times between two transmission pulses, the reflection signals of the preceding transmission pulses are detected by the receiver unit as received signals (alternating transmission mode and receiving mode). The distance to the target object(s) is determined by a direct (signal) propagation time measurement and the desired resolution (distance resolution) of the pulse radar system can be set by means of the pulse-on time (pulse width) of the transmission pulses. For distance selection when processing the received signal, "distance goals" are generally used corresponding to various signal pulse-on times and thus selective for a quite specific distance. PA1 In the FMCW radar system, the transmission signal is emitted continuously from the transmitter unit ("continuous wave" cw), while the transmission frequency of the transmission signal is varied, i.e. it has a specific modulation curve due to frequency modulation (FM), and at the same time the received signal is detected by the receiver unit. PA1 on account of the alternation (especially at high switching frequency) between transmission mode and receiving mode and the switching off of receiver unit and transmitter unit, it is possible to decouple transmission and reception in terms of time and cross-talk between transmitter unit and receiver unit is prevented; PA1 on account of the long pulse-on time of the transmission pulses and on account of the signal processing of the received signal that is equivalent to that of a FMCW radar system, little signal processing effort is required and good distance resolution is possible; PA1 only a single antenna is needed that can be used for both transmission mode and receiving mode at the same time; PA1 for the transmission pulses a variable and especially a large ratio of pulse-on time to pulse-off time can be predetermined (and thus a higher duty cycle), resulting in a high mean transmission performance of the radar system.
In the pulse radar system, it is advantageous that on the one hand transmission/receiving decoupling is easily possible, i.e., cross-talking of the transmission signal into the receiver unit can be prevented completely by suitably switching over from transmission to receiving mode (e.g., by means of transmission/receiving switches), and that on the other hand by presetting a gain related to the distance when processing a signal by controlling as a function of the propagation time the sensitivity of the signal gain ("Sensitivity Time Control" STC) the dynamic range of the received signal to be processed (the input dynamic response on detection of the received signal) can be reduced considerably.
The complexity of the pulse radar system and the associated high costs are disadvantageous because on the one hand the distance resolution depends solely on the pulse-on time and therefore with the very short pulse-on times needed to realize a high distance resolution (e.g., 1 m) a very fast signal processing of the received signal is also necessary (e.g., by digitally sampling the received signal by means of analog-to-digital converters with high clock frequency), and because on the other hand the signal processing of the received signal is very elaborate and furthermore is determined by the bandwidth of the transmission signal.
It is an advantage in the FMCW radar system that signal processing of the received signal is a relatively simple matter, even when the transmission signal has a large bandwidth, and that the distance and the relative velocity of the target objects (reflection objects) situated in the observation range can be determined with great accuracy and with high resolution in each case.
A disadvantage of the FMCW radar system is that, because the transmitter unit and the receiver unit are operating concurrently (concurrent transmission mode and receiving mode), a complex solution is required for adequate transmission-reception decoupling. This transmission-reception decoupling is realized, for example, either by installing separate antennas for transmission and reception or by using high class and thus costly components, in particular of couplers, which on the one hand results in high costs and on the other hand disturbing secondary effects (e.g. due to the non-ideal characteristics of the antennas or components).